Biologically active proteins that are microbially produced via recombinant DNA (rDNA) technology may contain cysteine residues that are nonessential to their activity but are free to form undesirable intermolecular or intramolecular links. One such protein is microbially produced human beta interferon (IFN-.beta.). In the course of the preparation of IFN-.beta. by rDNA techniques, it has been observed that dimers and oligomers of microbially produced IFN-.beta. are formed in E. coli extracts containing high concentrations of IFN-.beta.. This multimer formation renders purification and separation of IFN-.beta. very laborious and time-consuming and necessitates several additional steps in purification and isolation procedures such as reducing the protein during purification and reoxidizing it to restore it to its original conformation, thereby increasing the possibility of incorrect disulfide bond formation. Furthermore, microbially produced IFN-.beta. has also been found to exhibit consistently low specific activity due perhaps to the formation of multimers or of random intramolecular disulfide bridges. It would be desirable, therefore, to be able to alter microbially produced biologically active proteins such as IFN-.beta. in a manner that does not affect their activity adversely but reduces or eliminates their ability to form intermolecular crosslinks or intramolecular bonds that cause the protein to adopt an undesirable tertiary structure (e.g., a conformation that reduces the activity of the protein).
The present invention is directed to producing by directed mutagenesis techniques mutationally altered biologically active proteins (such proteins are called "muteins", Glossary of Genetics and Cytogenetics, 4th Ed, p 381, Springer-Verlag (1976)) that retain the activity of their parent analogs but lack the ability to form intermolecular links or undesirable intramolecular disulfide bonds. In this regard Shepard, H. M., et al, Nature (1981) 294:563-565 describe a mutein of IFN-.beta. in which the cysteine at position 141 of its amino acid sequence (there are three cysteines in native human IFN-.beta. at positions 17, 31, and 141, Gene (1980) 10:11-15 and Nature (1980) 285:542-547) is replaced by tyrosine. This mutein was made by bacterial expression of a hybrid gene constructed from a partial IFN-.beta. cDNA clone having a G.fwdarw.A transition at nucleotide 485 of the IFN-.beta. gene. The mutein lacked the biological activity of native IFN-.beta. leading the authors to conclude that the replaced cysteine was essential to activity.
Directed mutagenesis techniques are well known and have been reviewed by Lather, R. F. and Lecoq, J. P. in Genetic Engineering Academic Press (1983) pp 31-50. Oligonucleotide-directed mutagenesis is specifically reviewed by Smith, M. and Gillam, S. in Genetic Engineering: Principles and Methods, Plenum Press (1981) 3:1-32.